Description

Total points: 6 + 1 bonus points
Create a shell program (6 pt)

Use the Unix environment you installed in the previous homework to write a C or C++ program called mysh replaces the command shell in Unix. After started, it prints a prompt “#” and reads a command line terminated by newline. This line should be parsed out into a command and all its arguments. In other words, tokenize it.
• You do not need to handle “special” characters. Do not worry about handling quotation marks, backslashes, and tab characters. This means your shell will be unable support arguments with spaces in them. For example, your shell will not support file paths with spaces in them.
Implement built-in commands

Your shell should implement the following commands

# movetodir directory

Note: do not use the chdir() function call here!!!

# whereami

Prints the value of the currentdir variable.

# history [-c]

Without the parameter, prints out the recently typed commands (with their arguments), in reverse order, with numbers. For instance, it can print:

0: history 1: movetodir /etc 2: movetodir /eetc

If “-c” is passed, it clears the list of recently typed commands. You will need to implement an internal data structure to keep the list of commands. The data structure should allow you to grow the data structure to an arbitrary length.
You will need to save the history data structure to an external file (when exiting), and load it (when starting the shell).

# byebye

Terminates the mysh shell (and it should save the history file).

# replay number

Re-executes the command labeled with number in the history

# start program [parameters]

The argument “program” is the program to execute. If the argument starts with a “/” (such as /usr/bin/xterm, the shell should interpret it as a full path. Otherwise, the program will be interpreted as a relative path starting from the current directory.

The program will be executed with the optional “parameters”. It uses fork() + exec() to start the program with the corresponding parameters, and waits until the program terminates (use the waitpid() call).
For instance

run /usr/bin/xterm –bg green

would bring up a terminal with a green background. The prompt would not return until the terminal is closed.

Display an error message if the specified program cannot be found or cannot be executed.

# background program [parameters]

It is similar to the run command, but it immediately prints the PID of the program it started, and returns the prompt.

# dalek PID

Immediately terminate the program with the specific PID (presumably started from this command line interpreter). Use the kill() function call to send a SIGKILL signal to the program. Display success or failure.

Extra credit (1 point)

Implement a repeat command as follows:

# repeat n command …

Interprets n as the number of times the command must be run, command as the full path to the program to execute, and the others as parameters. The command should start the specified number of program instances, print the PIDs of the created processes and then return to the prompt, without waiting for the processes to terminate. For instance:
repeat 4 /usr/bin/xterm would bring up 4 terminals and print out something like:
PIDs: 31012, 31013, 31014, 31015.

More extra credit (1 point)

Implement the following command:

# dalekall

Which immediately terminates all the programs previously started by the mysh shell which had not been previously terminated by it, or by exterminate. It should output something like this:

Exterminating 3 processes: 16000 31012 31013
What to submit:

• The code as a single .c or .cpp file.
• If you implemented the extra credit part: a text file describing the syntax of the implementation, and example of use.

Notes:
Note 1: If you had not used a command line shell before, I would recommend that before you start the homework familiarize yourself with some shell(s). Bash – the default shell in most Linux installations and recent MacOS is a good choice. It is also available in Windows.

Note 2: Many of the individual components of this project are standard code snippets. Don’t be afraid to google for samples. For instance you might want to Google these and similar: o Tokenize string c++
o Fork process c++

Note 3: This is a relatively long, but easily modularizable program. This is an excellent opportunity to improve and show off your programming skills by organizing the program in a clever way. For instance, clearly, the individual commands should be implemented in their own function. Parameters should be passed to them. They should return an error / success code. The parsing should be done in one location, and the command functions called from there.
You should be able to debug these functions individually.

Note 4:
/usr/bin/xterm will work on Linux, or on Windows with VcXsrv or other X environment enabled. If you are on MacOS you will need to find a command line to bring up the Mac terminal.